Optoelectronic, high power, and high frequency devices are increasingly being fabricated using wide band gap compound semiconductor materials such as gallium nitride, aluminium nitride, and silicon carbide. Such semiconductor materials are frequently grown heteroepitaxially in thin film form onto a suitable substrate which provides a lattice matched template for crystal growth. Typical substrates include sapphire, silicon carbide, and silicon. For semiconductor devices such as microwave amplifier circuits, the substrate should be electrically insulating for the device to function.
A well known problem in semiconductor devices is that of heat dissipation. High temperatures often limit the performance and/or lifetime of such devices. This is a particular problem in semiconductor devices which operate at high power and/or high frequency such as microwave amplifiers, power switches and optoelectronic devices. It is therefore desirable to be able to spread any heat generated by component devices to reduce temperatures and thus improve device performance, increase lifetime, and/or increase power density. Accordingly, it is desirable to utilize a substrate material with a high thermal conductivity to spread the heat generated by a device, lowering the power density and facilitating dissipation via a heat sink thus improving device performance, increasing lifetime, and/or enabling an increase in power density.
Diamond has unique properties as a heat spreading material, combining the highest room temperature thermal conductivity of any material, with high electrical resistivity and low dielectric loss when in an intrinsic undoped form. Thus diamond is utilized as a heat spreading substrate for semiconductor components in a number of high power density applications. The advent of large area polycrystalline diamond produced by a chemical vapour deposition (CVD) technique has expanded the applications for diamond heat spreaders via an increase in area and a reduction in cost. A number of the favourable thermal, dielectric and insulating properties of diamond material are not exclusively available in naturally occurring or synthetic single crystal diamond material. Accordingly, polycrystalline CVD diamond wafers have been developed and are commercially available in sizes that enable them to be directly integrated with the fabrication processes of wide band gap semiconductors as a substrate material.
In light of the above, it is evident that for thin film compound semiconductor materials, an ability to integrate diamond as a carrier substrate could greatly improve thermal performance. For high power devices, the challenge is to position an active region of a device in as close proximity as possible to the heat spreading diamond substrate, since any intermediate carrier substrate material such as sapphire, silicon, or silicon carbide acts as a thermal barrier.
Compound semiconductor materials can be grown directly on a polycrystalline diamond substrate using, for example, a metal organic vapour phase epitaxy (MOVPE) technique. However, compound semiconductor material grown in such a manner will itself be polycrystalline, the crystals being distributed over a range of crystallographic orientations relative to the plane of the substrate. Such a polycrystalline layer of compound semiconductor material will tend to have relatively low charge mobility and thus will not provide good device performance for many proposed applications, particularly those which require high charge (electron and/or hole) mobility characteristics such as a high electron mobility transistor (HEMT) used in microwave frequency amplifier circuits. As such, it is desirable to provide a method which allows the provision of a monocrystalline compound semiconductor layer in combination with a polycrystalline diamond layer which functions as a heat spreading substrate. Routes to achieving a composite structure comprising a monocrystalline compound semiconductor layer in combination with a polycrystalline diamond layer may be split into three main categories:                (i) Forming a substrate comprising a monocrystalline compound semiconductor layer, forming a separate substrate comprising a polycrystalline diamond layer, and attaching the two substrates together using an adhesive in order to form a composite structure comprising a monocrystalline compound semiconductor layer in combination with a polycrystalline diamond layer. One problem with this method is that the adhesive used to bond the two substrates together can degrade during use due to heating leading to delamination. A further problem is that common adhesives do not have good thermal conductivity leading to poor thermal contact between the monocrystalline compound semiconductor layer and the polycrystalline diamond layer.        (ii) Growing a layer of monocrystalline compound semiconductor material on a substrate comprising a polycrystalline diamond layer. This may be achieved by forming a polycrystalline diamond layer with a thin layer of monocrystalline material thereon which is suitable for epitaxial growth of a monocrystalline compound semiconductor. For example, a layer of polycrystalline diamond can be grown on a monocrystalline silicon or silicon carbide substrate using a CVD technique. The monocrystalline silicon or silicon carbide substrate can then be processed such that only a thin layer of material remains adhered to the polycrystalline diamond layer. A layer of monocrystalline compound semiconductor material can then be epitaxially grown on the thin layer of monocrystalline silicon or silicon carbide. This results in a composite structure comprising a layer of polycrystalline diamond material, a layer of monocrystalline compound semiconductor material, and a thin intermediate layer of monocrystalline silicon or silicon carbide. The layer of monocrystalline silicon or silicon carbide should be as thin as possible to provide good thermal contact between the monocrystalline compound semiconductor layer and the polycrystalline diamond layer. Prior art documents relevant to this approach include: US 2006/0113545; US 2009/0272984; U.S. Pat. No. 7,695,564; and WO 2006/100559. One problem with this approach is that reducing the thickness of the silicon or silicon carbide wafer can be time consuming and/or difficult to control in order to provide a very thin layer of monocrystalline silicon or silicon carbide over the polycrystalline diamond substrate. Furthermore, the thin layer of monocrystalline silicon or silicon carbide can be subject to polishing damage, cracking, and/or delamination such as via peeling. As such, the quality of moncrystalline compound semiconductor material grown on such a thinned layer can be compromised and/or the intermediate layer is not sufficiently thin as to provide the desired level of thermal contact between the monocrystalline compound semiconductor layer and the polycrystalline diamond layer.        (iii) Growing a polycrystalline diamond layer on a substrate comprising a monocrystalline compound semiconductor material. This may be achieved by epitaxially growing a monocrystalline compound semiconductor material on a suitable carrier substrate such as monocrystalline silicon, silicon carbide, or sapphire. An interface layer can then be grown on the monocrystalline compound semiconductor layer, the interface layer forming a growth surface suitable for growth of synthetic diamond material thereon via a CVD technique. A CVD diamond growth process can then be used to form a layer of polycrystalline diamond material over the interface layer. This results in a composite structure comprising a layer of polycrystalline diamond material, a thin intermediate layer which may be, for example, silicon nitride, aluminium nitride, or silicon carbide, a layer of monocrystalline compound semiconductor material, and a layer corresponding to the original carrier substrate on which the monocrystalline compound semiconductor material was grown. The layer corresponding to the original carrier substrate may be partially or wholly processed off during further fabrication steps to yield a semiconductor device component. As before, the intermediate layer should be sufficiently thin to provide good thermal contact between the monocrystalline compound semiconductor layer and the polycrystalline diamond layer. Prior art documents relevant to this approach include: U.S. Pat. No. 7,939,367; US 2006/0266280; and US 2010/0001293. One problem with this approach is that thermal loading of the compound semiconductor substrate during CVD diamond growth thereon can damage the compound semiconductor material and/or result in the substrate warping (plastically deforming) on cooling which impairs the performance of the semiconductor substrate when incorporated into a semiconductor device.        
Embodiments of the present invention are concerned with the approach outlined in point (iii) above and particularly directed to solving the problems associated with warping of the compound semiconductor substrate. In this regard, U.S. Pat. No. 7,939,367 describes that the problem of substrate warping is due to temperature gradients across the substrate during CVD diamond growth thereon. It is described that the temperature difference between an edge and a centre of the substrate may be maintained within 80° C. by controlling the power ramping rate during CVD diamond growth of at least the initial layer of CVD diamond material. However, the present inventors have found that even if the power ramping rate is controlled in the described manner, warping and plastic deformation of compound semiconductor substrates is still problematic. This problem is exacerbated if the CVD diamond material grown thereon is grown under conditions suitable to obtain good CVD diamond growth rates and/or good quality polycrystalline CVD diamond material which has very high thermal conductivity.
In light of the above, it is an aim of certain embodiments of the present invention to address these problems.